Overshadowed by the mighty EF4 and EF3 tornadoes that tore across south-central Oklahoma last Monday, May 9, another less damaging twister--from the same thunderstorm--has left seasoned scientists both astonished and fascinated. This tornado, which struck near Roff, OK, was rated an unexceptional EF1 on the Enhanced Fujita Tornado Damage Scale. However, the Roff tornado stands out in several other ways.

—It was an anticyclonic tornado, meaning that it rotated clockwise rather than counterclockwise. A few such twisters probably occur each year, but they account for only perhaps 1% of all U.S. tornadoes—and that’s a very rough estimate, according to tornado climatology expert Harold Brooks (NOAA National Severe Storms Laboratory). In fact, the Storm Data procedures maintained by NOAA include no specific requirement or methodology for reporting whether or not a tornado is anticyclonic.

—It developed in the “forward flank” part of the storm, enveloped in rain-cooled air. “This is the first time I’ve ever heard of a well-developed anticyclonic tornado buried inside the forward-flank core,” said Roger Edwards (NOAA Storm Prediction Center), who has predicted and observed tornadoes for more than 25 years.

—Its damage path was initially estimated to be as much as 13 miles long, as documented in a preliminary storm survey conducted by the National Weather Service in Norman, OK. If confirmed, this path length will likely be a record; I have been unable to find evidence of any other anticyclonic tornado with a path this long. The estimated duration of 35 minutes might also end up as a record-setter for anticyclonic tornadoes. However, reanalysis of the damage could reduce the estimated path length and duration, as the radar signature of the tornado was shorter-lived, according to Gabriel Garfield (University of Oklahoma/CIMMS/NWS), who participated in the storm survey.

he scarcity of anticyclonic tornadoes is not a direct result of the Coriolis effect, which makes tropical cyclones spin cyclonically. The Coriolis force actually has little direct influence on circulations as small as tornadoes (not to mention toilets or other drains that happen to cross the equator). What appears to be the main driver is the wind shear that produces rotating supercell thunderstorms. In the Northern Hemisphere, a blossoming supercell will often split in two, with one cell angling to the right of the mean upper-level wind, spinning cyclonically, and the other angling leftward and rotating anticyclonically. Rightward-moving, cyclonically-spinning cells are the ones better positioned to ingest warm, moist air and grow more vigorously. Thus, most tornadoes are cyclonic, spawned by mesocyclones within cyclonically rotating supercells. (This Wikipedia page includes a nice conceptual diagram of a supercell thunderstorm.) Toward the outer edge of a storm's rear-flank gust front, there can be anticyclonically rotating features; very rarely, one of these will spin up an anticyclonic twister.

What happened in Oklahoma last week doesn't quite fit the classic picture: the anticyclonic Roff tornado developed right in the rain-cooled heart of the storm rather than on its periphery. The Roff tornado was embedded in rain for most or all of its lifespan. "We saw no evidence of it in the gray murk of heavy rain to our [west], nor any suspicious wind shifts," reported Edwards, who was on the same storm photographing the nearby Sulphur tornado. As for what caused the anticyclonic Roff tornado, "we can offer only speculation and conjecture at this point," said Edwards in a blog post. The smoking gun could end up being a stray left-moving storm, evident on radar, that zipped northward and became embedded in the larger, stronger supercell that produced the Roff and Sulphur tornadoes. It's conceivable that this left-moving cell injected some anticyclonic spin into the forward flank of the storm.

Deducing what happened will take some doing, as this storm was more than 50 miles from the Oklahoma City NEXRAD radar. Data from mobile radar, including the RaXPol unit operated by Howard Bluestein and colleagues (University of Oklahoma), may shed some light on the oddly positioned Roff twister. "Having a tornado in that location is worrisome for my team," said Doppler on Wheels chief scientist Joshua Wurman in an email, "because we count on being able to transect cores safely. That's not where we expect to find danger."

Edwards mused: "The odds are extraordinarily tiny of ever seeing another anticyclonic tornado entombed in the forward-flank core of even a violently tornadic supercell. Yet now we know it's possible, and we must be vigilant of that."

Some clockwise pioneersThere are extremely rare cases where a left-moving, anticyclonically rotating thunderstorm generates an "anti-mesocyclone" that spawns an anticyclonic twister, as occurred near Sunnyvale, CA, on May 4, 1998. However, the best-known examples of anticyclonic twisters occur in conjunction with more powerful cyclonic twisters as part of a single supercell thunderstorm, as was the case in Oklahoma last week. Sometimes these are weak, short-lived satellite twisters very close to the companion cyclonic tornado; others are stronger and more separated. Wurman unraveled DOW data for several of these in a 2013 Weather Analysis and Forecasting paper. "Our paper does not include any observations of [forward-flank] anticyclonic tornadoes, so this current one is certainly an outlier," Wurman told me.

One classic case--probably the first anticyclonic tornado ever filmed--unfolded just west of Ames, IA, on June 13, 1976. In this storm, a powerful F3 anticyclonic tornado (the strongest anticyclonic tornado documented to date) closely paralleled the path of an F5 that ripped through the town of Jordan and the nearby countryside. The twisters were only about a mile apart as they moved largely in sync (see Figure 3). The YouTube clip at bottom shows both tornadoes.

A few years later, Ted Fujita and Roger Wakimoto examined the mammoth, nearly stationary supercell that tormented Grand Island, NE, for nearly three hours on June 3, 1980. (This is the storm that inspired the book and movie "Night of the Twisters"). Of the seven tornadoes that emerged that night, three were found to be anticyclonic. "This storm produced the most complex damage patterns imaginable," wrote James McDonald (Texas Tech University) in the Bulletin of the American Meteorological Society. "No one but Ted Fujita could have sorted them out."